1. Field of the Invention
The present invention relates to a wheel bearing assembly for use in automotive vehicles and freight cars, which has an improved strength.
2. Description of the Related Art
In some of the wheel support bearing assemblies of the second and third generation types, in which an inner member is used on a rotatable side and an outer member is used on a stationary side, a vehicle fitting flange, through which the wheel support bearing assembly is secured to an automotive vehicle, has hitherto been provided on an outer periphery of the outer member. Because of the provision of the vehicle fitting flange, the required strength has been secured by the use of a medium carbon steel (C=0.5 to 0.8 wt %) such as, for example, S53C as material for the outer member in terms of the processability during the hot forging. Although the rolling surface in the outer member is partially hardened by means of the induction heating treatment in order for it to have a required hardness as a rolling area of the bearing assembly, other portions than the rolling area are left untreated thermally and they are used as forged or non-hardened.
Also, even the inner member (hub, hub ring) of the wheel support bearing assembly, which serves as a rotatable member, and the outer member in some of the wheel support bearing assemblies of the second generation type, in which the outer member is used as a rotatable member, are made of a medium carbon steel in a manner similar to that described above, and the rolling surface is partially hardened by means of the induction heating treatment whereas the other portions than the rolling area are left untreated thermally and they are used as forged.
Demands for reduction in weight of the wheel support bearing assembly in the course of reduction in weight of the automotive vehicle are now increasing, and even the wheel support bearing assembly, which has been reduced in weight and size, is required to have a functionality similar to that afforded by the conventional bearing assembly. For example, to meet with the demands for reduction in weight (in volume), it is quite often that the vehicle fitting flange integral or rigid with the outer member serving as the stationary member is downsized and/or thin walled.
It has, however, been found that if the vehicle fitting flange integral or rigid with the outer member is downsized and/or thin walled, stresses generated in various parts of the wheel support bearing assembly during the cornering of the automotive vehicle tend to become high and, therefore, the strength properties against repeated loading is required. To satisfy the strength properties, it has been well known in the art to thermally refining the outer member by means of quenching and tempering to thereby increase the tensile strength and the fatigue strength. Also, even in the case of the rotatable member (hub ring, or the outer member of an outer ring rotating model of the wheel support bearing assembly of the second generation type), the rotatable member in its entirety is thermally refined to increase the fatigue strength of a prominently stressed site during the cornering of the automotive vehicle. (See, for example, the Patent Documents 1 and 2 listed below.)    [Patent Document 1] JP Laid-open Patent Publication No. 2005-003061    [Patent Document 2] JP Laid-open Patent Publication No. 2007-211987
The thermal refinement by way of quenching and tempering is considered the best technique to increase the mechanical characteristics such as, for example, toughness and fatigue strength if the member is transformed completely into the martensite structure during the quenching and then into the sorbite (micro-pearlite) structure by means of tempering.
However, in the face of limitations in shape and size of the member to be thermally refined, it has been found that an attempt to quench it in water, which results in a quick cooling, to transform completely into the martensite structure tends to result in quench cracking and/or quench deformation.
Also, when the member is transformed completely into the martensite structure in not only a surface region thereof, but deep into the core region thereof, the residual stress in circumferential and axial directions of the member surface gives rise to the tensile stress, which in turn becomes a high tensile stress when combined with a stress generated by the loading of a load. As a result, the member becomes susceptible to cracking, which is apt to propagate.